In a large number of medical procedures, at least a portion of the blood volume is passed through a filter. The filter is designed to remove certain particulate matter from the blood. Alternatively, the filter may be designed to remove specific chemicals and water solutions during the filtering process.
A typical device that exposes blood to a filter is a dialyzer. One of the main parameters of filter quality is filter blood volume (FBV). In terms of the dialyzer, the filter blood volume is often referred to as dialyzer blood volume (DBV). Alternatively, the volume has been referred to as fiber bundle volume. The term filter blood volume (FBV) will be used to encompass all the filter constructions including filters having plates, fibers or biological cells.
Due to the exposure to the blood, clotting may occur in the filter and thereby significantly decrease the surface area of exchange available to the blood and hence decrease filtration. In this case the quality of treatment may be jeopardized. Fiber clotting during hemodialysis can significantly decrease the volume of blood for solute exchange.
Filter bundle volume, the total space within the blood compartment of hollow fiber hemodialyzers, correlates closely with dialyzer surface area, a major determinant of solute clearance. A decrease in surface area due to clotting causes a decrease in solute clearance that puts the patient at risk for inadequate dialysis. Since FBV correlates with membrane surface area and is easy to measure in vitro, FBV has been selected, in centers where dialyzer reuse is permitted, as the main criterion that allows a dialyzer to be reused. In countries where reuse of dialyzers is permitted, the main criteria for reuse is comparison of current FBV to its initial value. Traditionally, FBV is measured after cleaning, clot removal, and pressure flushing. The FBV values determined by this traditional process may not represent the actual FBV.
The sequence of procedures for dialyzer reuse consists of cleaning, including flushing the fibers with pressurized water to remove clots and debris, and measurement of FBV by volumetric displacement of air or liquid which is compared with the original dialyzer volume. Because vigorous flushing removes much of the clot, FBV measured in vitro may not represent the true in vivo volume and the corresponding surface area available for solute exchange.
As the filters must be monitored and changed, the filters are usually disposed extracorporeal. The relationship of a filter to a particular medical procedure is sensitive to a variety of parameters. Generally, the operating pressure, or resistance of a filter must be within particular predetermined limits.
However, as the filter is often removing material that is not perceptible to the naked eye, it is general practice to remove the filter from the fluid circuit and fill the filter with water to determine the filter volume. This process is not only time consuming, but exposes the filter to contamination. Also, the interruption of the filtering process can be detrimental to the treatment of the patient. For example, in procedures such as hemodialysis, a filter is used to remove selected particles and liquids from the bloodstream of the patient.
Difficulties in making such measurements have resulted from the fact that such procedures usually involve extracorporeal circulation of the blood from a patient through, for example a blood treatment system, and in many cases the effects of the system itself on the blood flow or on the measurement devices is unknown. For example, if blood is directed to a dialysis filter through plastic tubing, the effect of the plastic material on measuring equipment using ultrasound waves may not be known with any certainty, since characteristics of the material can vary from one tube to another.
There is emerging technology of locating a blood filter inside the patient. However, even these filters are subject to the traditional concerns of proper functioning. If the filter is inserted into the body, then the measurements are made using sensors mounted on a blood vessel.
Therefore, the need exists for a method and apparatus for monitoring the FBV. A need also exists for the real time monitoring FBV. A need further exists for measuring the FBV during use or when the filter is not in use, wherein the FBV volume has improved accuracy. The need also exists for monitoring a change in the FBV to allow adjustments to the procedures to accommodate or correct such volume changes.